1. Field of Invention
This invention relates generally to electronic test and measurement equipment and more specifically to the measurement of jitter.
2. Discussion of Related Art
Jitter is a characteristic of periodic signals that is often undesirable. If a signal is perfectly periodic, it will repeatedly take on the same value at points in time that are spaced by exactly the period of the signal. Jitter is the differences between the actual time at which the value recurs and the nominal times at which it should recur in a perfectly periodic signal.
Jitter might be introduced into a signal by many sources, including electrical interference that creates noise. Approximations in representing signal values and other errors in a circuit might all contribute to jitter.
Some amount of jitter is unavoidable in every signal. If the jitter is a relatively small fraction of the period of the signal, it is unlikely to impact the operation of electronic circuits that operate on the signal. However, some circuits are designed assuming that the signals they process have a specific period or take on specific values at defined times. If there is too much jitter in these signals, the circuits might fail to operate properly.
A desirable attribute of certain electronic components is the ability to operate even when input signals have jitter. Many standards for communication protocols such as IEEE 802.3ae for XAUI and 10G Ethernet impose requirements that can only be met if communication circuits operate in the presence of jitter. An engineer designing a communications system, for example, might wish to know the jitter immunity of a semiconductor device containing a receiver to determine whether the system will operate in compliance with the specification. To enable the engineer to make this determination the jitter immunity of the semiconductor device including the receiver must be known. Accordingly, some semiconductor devices are sold with a jitter specification that indicates how much jitter might be present on inputs to the device and still have the device operate as expected or the maximum amount of jitter the device might have on its output.
Jitter immunity of a semiconductor device can be characterized using automated test equipment. The test equipment includes a signal source that can be programmed to generate periodic signals with a programmable amount of jitter, i.e. a “jitter injector.” The automated test equipment is generally constructed to determine whether a semiconductor device complies with applicable standards or otherwise operates as intended. During jitter characterization, jitter is intentionally introduced in a signal applied as a clock or other input to the device under test. The amount of jitter that causes the device to fail indicates its jitter immunity.
A similar setup can be used to test semiconductor devices as part of their production. The automated test equipment generates an input to the device under test with an amount of jitter equal to the specified jitter immunity of the device. If the device operates properly even with that level of jitter, it can be classified as a good device. Conversely, if it does not operate properly, the device might be rejected or “binned” as a part having a reduced jitter immunity specification.
For the characterization or test techniques above to be accurate, it is important that the jitter injector actually produces the exact amount of jitter it is programmed to produce. Periodically, the amount of jitter produced by a jitter injector might be measured and compared to the programmed amount. Such a process is known as verification.
Various methods to measure jitter are known, such as those specified in Annex 48B of the IEEE 802.3ae standard. For example, phase noise analyzers and real time oscilloscopes have been used to measure jitter. However, these devices often have limited bandwidth or frequency responses that make them unsuitable for high frequency measurements. However, jitter measurement is particularly important for very high frequency signals, such as those in the range of approximately 10 GHz.
Sampling oscilloscopes have also been used for jitter measurements. Sampling oscilloscopes generally have higher input bandwidth than a real time oscilloscope. The sampling oscilloscope might present the samples graphically as a waveform on a display or as a data file that can be processed in a computer or other data processing device.
It would be desirable to have more accurate jitter measurements techniques, particularly ones that are operable for measuring jitter on signals having a frequency between 1.5 and 12.5 GHz.